The present invention relates to a two-stroke cycle gasoline engine, and, more particularly, to a two-stroke cycle gasoline engine adapted for use with automobiles.
A two-stroke cycle engine has theoretically the advantage that an engine of a certain size can generate a greater power than a four-stroke cycle engine of a bigger size because the two-stroke cycle engine has twice as many work cycles per revolution as the four-stroke cycle engine. In fact, however, the conventional two-stroke cycle gasoline engine employing a carburetor has such drawbacks that it has high fuel consumption as compared with the four-stroke cycle engine due to the loss of air-fuel mixture caused by the direct escape, i.e. blow-out, of scavenging mixture to an exhaust manifold during scavenging, and that it cannot generate such a high power as expected from the fact that it has twice as many work strokes as the corresponding four-stroke cycle engine, due to the fact that the scavenging is still insufficient. Because of these problems, the practical use of two-stroke cycle gasoline engines is nowadays limited to the field of small engines which must be simple in structure and low in manufacturing cost.
Conventional two-stroke cycle gasoline engines of the abovementioned type, therefore, generally employ crankcase compression for scavenging. However, the scavenging by crankcase compression is not fully effective and can only provide a relatively low volumetric efficiency. This is the principal cause of the poor output power of conventional two-stroke cycle gasoline engines. In fact, a volumetric efficiency as high as 80% is available in four-stroke cycle engines, while on the other hand the volumetric efficiency of typical two-stroke cycle engines is still as low as 40-50%. The pump stroke volume of crankcase compression is equal to the stroke volume of the engine. However, since the crankcase has a relatively large clearance volume, the compression ratio of crankcase compression is relatively low, so that as a result the amount of air-fuel mixture drawn to the crankcase is small, the amount of delivered mixture is small, the delivery pressure is low and hence the scavenging pressure is low, and consequently it is hard to supply a really adequate amount of scavenging mixture into the power cylinder. As a result, the delivery ratio obtained in an engine wherein scavenging is effected only by the normal crankcase compression is only as high as 0.5-0.8. Since further the trapping efficiency is about 0.7, the volumetric efficiency becomes as low as 40-50% as mentioned above.
The purpose of scavenging is to push the residual exhaust gases in the power cylinder out of it by fresh mixture, and therefore if the pressure of the residual exhaust gases and the distance between the scavenging port and the exhaust port are given, the time required for completing scavenging is determined, provided that stratified scavenging is performed. Now, if the scavenging pressure is low, as when crankcase compression is used, a relatively long time is required for completing scavenging, particularly when the scavenging is performed by uniflow scavenging, and therefore, when the engine is rotating at high speed, it may well occur that the exhaust port is closed before the scavenging is completed so that a large amount of exhaust gases still remains in the power cylinder, and thereby only a very small amount of fresh mixture is charged into the power cylinder. Therefore, conventional two-stroke cycle engines have been unable to operate satisfactorily in the high speed range.
Therefore, it is an object of the present invention to provide a two-stroke cycle gasoline engine which produces a substantially higher power output per unit volume of the engine displacement when compared with the conventional two-stroke cycle gasoline engine by incorporating a special scavenging pump means in the engine in addition to or, alternatively, without depending upon the crankcase compression, thereby substantially increasing both the amount and the pressure of scavenging mixture so that the volumetric efficiency is increased up to 75-90% or in some cases exceeds 100%.
Another object of the present invention is to reduce the clearance volume of the scavenging means by employing a separate pump means other than the crankcase compression, thereby increasing the scavenging pressure so that the time required for scavenging is so shortened that the scavenging efficiency is increased up to 80-90% and that a high power operation of a two-stroke cycle gasoline engine is ensured even in a relatively high speed operational range.
However, it is to be noted that the relatively high-speed operational region contemplated in the present invention means such an operational region in which the conventional, particularly the uniflow scavenging type, two-stroke cycle gasoline engine is unable to operate with sufficient output power, due to insufficient scavenging at high rotational speed. In fact, the aforementioned relatively high speed rotational region is located in a lower speed region than the high rotational speed region of the conventional automobile four-stroke cycle gasoline engines, as explained below. Therefore, it is still another object of the present invention to provide a two-stroke cycle gasoline engine which can operate in such a lower speed operational region so as to generate sufficient output power. Conventionally, a relatively small-sized four-stroke cycle engine for automobiles is designed so as to be operated at relatively high rotational speed so that relatively high power output is available from a relatively small size engine. In this connection, it is noted that, for example, in the case of an engine which has a two liter piston displacement and produces 92 PS of brake horsepower at 5000 rpm, a very large proportion of the power, such as 52 PS out of the indicated horsepower of 144 PS, is consumed by internal friction losses in the engine. The ratio of the internal friction loss to the output power of the engine is substantially reduced by lowering the rotational speed of the engine. In view of this, still another object of the present invention is to utilize the advantage that the two-stroke cycle engine has twice as many work strokes as the four-stroke cycle engine by increasing the volumetric efficiency of the power cylinder, and to provide an engine which produces a high effective power output per unit stroke volume of the engine without increasing the rotational speed to such a high range as in conventional relatively small four-stroke cycle automobile engines. The maximum rotational speed of the engine contemplated in the present invention is 3800 rpm at the highest.
Methods of scavenging in two-stroke cycle engines are conventionally known as cross scavenging, loop scavenging, and uniflow scavenging. In this connection and in connection with the aforementioned high pressure scavenging contemplated in the present invention, if the scavenging pressure is increased in cross or in loop scavenging, the flow of scavenging mixture is liable to penetrate through the layer of exhaust gases existing in the power cylinder in a short-cutting manner, and also scavenging mixture and exhaust gases may be mixed with each other, thereby not only causing poor scavenging but also increasing the above explained blow-out loss of mixture, thus lowering the volumetric efficiency. On the other hand, it has been experimentally confirmed that when uniflow scavenging is employed, it is possible to push the exhaust gases existing in the power cylinder uniformly out of it by the scavenging mixture at high pressure without causing any detrimental mixing between the scavenging mixture and the exhaust gases, and that in this case if the amount of scavenging mixture is increased so as to be necessary and sufficient, scavenging at high scavenging efficiency is accomplished and, as a result, the volumetric efficiency increases, resulting in corresponding increase of engine output power.
Therefore, it is still another object of the present invention to provide a two-stroke cycle gasoline engine in which high pressure scavenging and uniflow scavenging are combinedly incorporated.
In order to accomplish the aforementioned various objects and features of the present invention, the present invention proposes, as one of its principal features, to incorporate, in a two-stroke cycle engine having at least one two-stroke cycle power cylinder-piston assembly, a scavenging pump means having a total stroke volume of between 1.35 and 1.85 times as much as that of the power cylinder-piston assembly. Such a scavenging pump means may include a pump mechanism depending upon the crankcase compression. In this case, since the stroke volume of the crankcase compression pump mechanism is equal to that of the power cylinder-piston assembly, an independent pump mechanism is required to have a total stroke volume of between 0.35 and 0.85 times as much as that of the power cylinder-piston assembly. In this connection, for a fixed stroke volume of the scavenging pump means, the scavenging pressure effected by the scavenging pump means changes as the clearance volume of the pumping chamber changes. Particularly the crankcase inevitably has a relatively large clearance volume, and therefore in conventional crankcase compression normally scavenging pressure of only about 0.3 atm (gauge pressure) is available. However, in some engines of improved designs, scavenging pressure of about 0.45 atm is available. By contrast, the present invention contemplates to increase the scavenging pressure at the starting of scavenging in high speed operation of the engine up to about 0.6 atm. The pressure of the residual exhaust gases in the power cylinder after the unaided exhaustion of exhaust gases is about 0.2 atm, and by employing scavenging pressure of the order of 0.6 atm the scavenging of the power cylinder is rapidly accomplished. When the crankcase compression is combined with an independent scavenging pump, the clearance volume of the crankcase affects the amount and the pressure of scavenging mixture. When the clearance volume of the crankcase is small, the stroke volume of the independent scavenging pump may be small, while on the other hand when the clearance volume of the crankcase is large, the stroke volume of the independent scavenging pump must be increased. When the scavenging pump means does not incorporate crankcase compression and is completely dependent upon an independent scavenging pump, since the clearance volume of the independent scavenging pump is generally small, the adjustment between the amount and the pressure of scavenging mixture may be made by a mixture tank connected to the delivery port of the scavenging pump. For example, if it is assumed that the delivery amount of a scavenging pump is 70% of its stroke volume, that crankcase compression is not incorporated and that the pump has a stroke volume of 1.85 times as much as the engine stroke volume, then the delivery amount of the pump is 1.3 times as much as the engine stroke volume. However, the mixture discharged from the pump is partly trapped in the mixture tank or passages, and all of the mixture delivered from the pump is not effectively used for scavenging. Therefore, it must be noted that even when the ratio of pump stroke volume to engine stroke volume is the maximum, i.e. 1.85, the delivery ratio is of the order of 1.3 or less. However, in accordance with the present invention, the delivery ratio is substantially larger than that available by conventional crankcase compression, which is of the order of 0.5-0.8.
Even when crankcase compression is employed in accordance with the present invention, it is desirable that the crankcase clearance volume is made as small as possible. Normally the crankcase clearance volume is 2-3 Vs, wherein Vs is engine stroke volume, and therefore the compression ratio is 1.5-1.3. However, if special designs such as involving filling up the back of the piston are incorporated, the clearance volume can be reduced to the order of 1.3 Vs, so that the compression ratio is 1.75. Therefore, when crankcase compression is combined with an independent scavenging pump of the minimum volume (0.35 Vs), if it is assumed that the total clearance volume is, for example, 2 Vs, a compression ratio of the order of 1.6 is available, i.e. (2+1.35)/2=1.675. Furthermore, if the delivery ratio available by crankcase compression only is 0.8, the delivery ratio is increased up to the order of 1.0, i.e. if the ratio of the delivery amount of the 0.35 Vs independent pump to the stroke volume thereof is 70%, 0.35 Vs.times.0.7=0.245 Vs, and this scavenging mixture having the volume of 0.245 Vs in the atmospheric condition is added to the amount of scavenging mixture obtained by the crankcase compression only. When crankcase compression is not combined with the independent scavenging pump of the present invention, the amount and the pressure of scavenging mixture are more freely designed than in the case of incorporating the crankcase compression, by adjusting the volume of a mixture tank and of scavenging passages.
The aforementioned condition with regard to the total stroke volume of the scavenging pump means, i.e. the ratio of between 1.35-1.85 of the total stroke volume of the scavenging means to that of the power cylinder-piston assembly, has been obtained from the above-explained considerations and experimental researches based upon those considerations. If the total stroke volume of the scavenging pump exceeds 1.85 times the total stroke volume of the power cylinder, even when the volumetric efficiency of the pump and the ineffective part of the delivered mixture which is trapped in the crankcase, in a mixture tank and in passages are taken into consideration, and even when uniflow scavenging is employed, the delivery ratio becomes so high that the blow-out of the mixture to the exhaust manifold increases up to an undesirable extent. On the other hand, the value of 1.35 for the ratio of the total stroke volume of the pump to the total stroke volume of the power cylinder is, in consideration of the volumetric efficiency of the pump and the ineffective part of the mixture delivered from the pump, the lower limit allowable for accomplishing the object of the present invention which is to obtain a high scavenging efficiency by uniflow scavenging coupled with high flow and pressure of scavenging mixture obtained by the combination of the crankcase compression having a relatively small clearance volume and an independent scavenging pump. When crankcase compression is not combined, the flow and the pressure of scavenging mixture required for accomplishing the objects of the present invention can be more freely determined within the range between the aforementioned upper and lower limits when compared with the case of combining crankcase compression.
Furthermore, currently there exists a great demand for the development of cars which have low fuel consumption, in view of energy saving. Furthermore, cars must satisfy a high standard with regard to the prevention of air pollution. In order to improve fuel consumption, not only the improvement of the fuel consumption of the engine itself but also the reduction of the weight and the air resistance of the vehicle are required. We noted, in connection with various running tests carried out to prepare for the qualification tests for conforming to the standards for the prevention of air pollution which are becoming more severe nowadays, that fuel consumption is different in summer and in winter due to the difference of atmospheric air density, and we more keenly recognized that the air resistance of the vehicle has an important effect on the fuel consumption of the vehicle even in low speed running. In order to lower the air resistance of the vehicle it is important to reduce the height of the vehicle as much as possible and to form the external shape of the vehicle in a streamlined shape. Particularly it is very effective to lower the engine hood. In order to reduce the height of the vehicle it is effective to eliminate the drive shaft for driving the rear wheels so that the shaft tunnel is eliminated and a flat floor is available, over the entire floor area, thereby constructing a vehicle body having a low floor and a low roof. A method for accomplishing this is to employ the FF system, i.e. the front engine-front drive system. In order to lower the engine hood by a large amount in an automobile of FF system while ensuring necessary leg room for the driver and the front seat passenger, it is necessary to reduce substantially the height and length of the engine compartment. Furthermore, in order to reduce the air resistance of the vehicle, it goes without saying that the frontal area of the vehicle must be reduced. Therefore, the width of the vehicle should be minimized. Furthermore, since the transmission, differential gears, and other driving mechanisms must be housed in the engine compartment together with the engine, in the FF system, the space allowed for the engine is much reduced. Light trucks are often designed with the engine mounted under the driver's seat, and in such a design the engine, being relatively long, often extends so far backward as to make a hump of the engine enclosure rearward of the cabin, thus shortening the deck.
It is therefore still another object of the present invention to deal with the aforementioned problems and requirements and to provide a small size gasoline engine having a low height, a small length and not a very large width yet being capable of generating high power.
As uniflow scavenging engines are known an engine having horizontally opposed pistons, an engine having an exhaust poppet valve, etc. In order to accomplish the aforementioned objects of the present invention, we now consider an engine having horizontally opposed pistons. That is, it is found that an engine having a power cylinder-piston assembly employing horizontally opposed pistons is particularly advantageous.
Therefore, in order to accomplish the aforementioned object, the present invention proposes to employ at least one two-stroke cycle power cylinder-piston assembly incorporating uniflow scavenging and two horizontally opposed pistons as the power cylinder-piston assembly of the engine. By combining such a power cylinder-piston assembly with the aforementioned concept of high flow and pressure of scavenging mixture, it is possible to charge the power cylinder with fresh mixture with high volumetric efficiency without causing substantial blow-out of scavenging mixture to the exhaust manifold, and because of this it is possible to obtain an engine of reduced height and length having the high power generating ability even at a relatively low rotational speed, when compared with a conventional four-stroke cycle engine. Furthermore, in contrast to the emission performance of the conventional two-stroke cycle gasoline engine, which shows high concentration levels of HC in the exhaust gases, such as 5-10 times as high as those of the conventional four-stroke cycle gasoline engine, the engine of the present invention is able, due to substantial avoidance of blow-out of scavenging mixture to the exhaust manifold, to keep HC concentration in the exhaust gases at a sufficiently low level.
In connection with the aforementioned concept of employing at least one two-stroke cycle power cylinder-piston assembly incorporating uniflow scavenging and two horizontally opposed pistons as the power cylinder-piston means of the engine, the present invention further proposes to employ at least one pump cylinder-piston assembly of the reciprocating type as the scavenging pump means. By employing such a pump cylinder-piston assembly it is possible to secure the necessary amount and pressure of scavenging mixture even in low speed operation and it is also possible to construct the scavenging pump with a simpler and less expensive structure. When compared with this, if a rotary pump is employed, scavenging pressure is constantly applied to the scavenging port even during the non-scavenging period, whereby it happens that scavenging mixture leaks through the clearance between the power cylinder and the piston, thereby increasing the pumping loss. Furthermore, if the rotary pump is a centrifugal pump, although its structure is simple, there is the problem that the flow of scavenging mixture is insufficient in starting and low speed operations. In contrast to the rotary pump, the reciprocating pump is readily adapted with a proper phase difference to be synchronized with the power cylinder-piston assembly, so that the required scavenging pressure is generated only when the power cylinder-piston assembly is to be scavenged. In this connection, furthermore, if a pump cylinder-piston assembly incorporating horizontally opposed pistons is employed as a reciprocating pump in combination with the aforementioned two-stroke cycle power cylinder-piston assembly incorporating uniflow scavenging and horizontally opposed pistons, another advantage is obtained in that more desirable harmony between the dimensions of the power cylinder-piston assembly and of the pump cylinder-piston assembly is available.
In more detail, a two-stroke cycle power cylinder-piston assembly incorporating uniflow scavenging and horizontally opposed pistons has a volume to be scavenged slightly more that twice as much as the stroke of the individual pistons. Therefore, if the power cylindermust be scavenged by a scavenging pump having a single piston, either the diameter of the pump cylinderor the stroke of the pump piston must be relatively large. In either case, in view of the fact that the total stroke volume of the scavenging pump means is to be 1.35-1.85 times as large as the total stroke volume of the power cylinder-piston assembly, particularly when crankcase compression is not employed, it is apprehended that either the width or the length of the scavenging pump means may become too large compared with those of the power cylinder-piston assembly. However, if the scavenging pump means is provided as a pump cylinder-piston assembly having horizontally opposed pistons, it is possible to maintain both the diameter of the pump cylinderand the stroke of the pump piston within reasonable values so as to provide desirable harmony with the power cylinder-piston assembly. When such a pump cylinder-piston assembly is arranged horizontally side by side with a power cylinderpiston assembly of the same type having horizontally opposed pistons, the engine presents a compact overall configuration like a horizontally flat block, rectangular in a plan view.
An engine for a small size or light automobile will comprise, at the most, one or two two-stroke cycle power cylinder-piston assemblies of the aforementioned type incorporating uniflow scavenging and horizontally opposed pistons. In this case the balancing of the scavenging pump is important. Even when an independent scavenging pump is relatively small due to use of crankcase compression, if one power cylinder-piston assembly incorporating uniflow scavenging and horizontally opposed pistons is served by a single cylinder-single piston scavenging pump, the pump piston will become relatively large, requiring a relatively large counterweight, resulting in a relatively large crankcase, yet perfect balancing of reciprocating masses will not be attained. In this respect, if the pump is a cylinder-piston assembly having horizontally opposed pistons, the inertia forces of the reciprocating masses related to individual opposed pistons are perfectly balanced, whereby the crankcases for individual pistons are substantially reduced in size together with reduction of the height and length of the engine, thereby providing a compact two-stroke cycle engine of the horizontally opposed piston type less prone to vibration.
However, the differences of engine volume and of dynamic balance between a single piston scavenging pump and an opposed piston scavenging pump will become less important as the engine becomes smaller, while on the other hand, if the engine becomes smaller, the difference in manufacturing cost, which is governed by structural complexity, will become more important. Therefore, it must be individually examined according to various conditions which of the two factors should have priority over the other.
When a pump cylinder-piston assembly having horizontally opposed pistons is employed as the scavenging pump means, the reciprocating inertia forces in the pump means are well balanced, and this, in combination with a power cylinder-piston assembly of the same horizontally opposed piston type in which the reciprocating inertia forces are also well balanced, can provide a well balanced, less prone to vibration, and quiet engine.
With respect to a pair of crankshafts of the power and pump cylinder-piston assemblies of the horizontally opposed piston type, if they are rotated in opposite directions, moments produced by forces perpendicular to the crankshafts are also balanced. However, this requires incorporating a rotation reversing mechanism including an idle gear between the two crankshafts, and therefore increases manufacturing cost. Therefore, as an embodiment of the present invention, it is proposed to drivingly connect a pair of crankshafts of the power and pump cylinder-piston assemblies of the horizontally opposed piston type simply by an endless chain so that the two crankshafts rotate in the same direction. In this regard, it is a matter of choice between pursuing quietness of vibration in engine operation and pursuing reduction of cost to select the system of mutual counter-rotation of a pair of crankshafts or to select the system of rotation in the same direction of a pair of crankshafts, and this is, in any event, a matter of design with regard to the engine of the present invention.
When the two-stroke cycle gasoline engine of the present invention comprises, for example, two two-stroke cycle power cylinder-piston assemblies, and if the crankcases of these power cylinder-piston assemblies are not utilized for crankcase compression of scavenging mixture, the scavenging pump means to serve for the two power cylinder-piston assemblies must have a relatively large capacity. Therefore, even when the scavenging pump is constructed as a pump cylinder-piston assembly having horizontally opposed pistons, a single acting pump cylinder-piston assembly of the horizontally opposed piston type will not be sufficient to supply the necessary flow of scavenging mixture. Furthermore, when two power cylinder-piston assemblies are combined to operate with phase difference of 180.degree. therebetween, another difficulty is encountered with regard to the matching of the operational phase of the scavenging pump to that of the power cylinder-piston assemblies. In view of these problems, the present invention further proposes to employ a double acting pump cylinder-piston assembly having horizontally opposed pistons so as to make the two actions of the pump pistons serve for the scavenging of the first and second power cylinder-piston assemblies, respectively. By this arrangement, it is possible to supply scavenging mixture to two power cylinder-piston assemblies by using one pump cylinder-piston assembly while maintaining harmony between the dimensions of the power cylinder-piston assemblies and of the pump cylinder-piston assembly, and thus an engine having high power output relative to its volume is obtained.
It is not indispensable that the power cylinder-piston assembly of the uniflow scavenging and horizontally opposed piston type and the pump cylinder-piston assembly incorporated in combination in the engine of the present invention should be synchronized with each other with a strict 180.degree. phase difference therebetween in a manner such that when the power piston is in its bottom dead center, the pump piston is in its top dead center. On the contrary, it is possible to obtain a more desirable effect by modifying the phase difference between the power and pump pistons slightly from 180.degree.. In more detail, by shifting the phase of the pump piston relative to the power piston so that, when the power piston is in its bottom dead center, the pump piston is slightly before its top dead center, by, for example, about 15.degree., i.e. by retarding the phase of the pump piston slightly more than 180.degree. from the power piston, the performance of scavenging in the scavenging period in which the power piston has passed its bottom dead center can be somewhat improved.
In the present description, the top dead center (TDC) of the pump piston means the dead center of the pump piston at the end of pump compression stroke. As a conventional technique for supplementing insufficiency of scavenging by the crankcase compression, it has been proposed to employ a stepped piston. In this case, however, due to incorporation of a scavenging pump cylinderbetween the power cylinderand the crankcase, the length of the cylinder-piston assembly is substantially increased, and the weight of the piston is also increased, thereby causing, as a matter of course, weight increase of the connecting rod and of the crank arm, thereby increasing the weight of the counterweight, which causes a size increase of the crankcase. Therefore, a power cylinder-piston assembly incorporating uniflow scavenging and horizontally opposed stepped pistons will have an abnormally large width when it is used as an automobile engine and will not be suitable for being mounted in the engine compartment of a small size or light automobile. Further, in the present description the total stroke volume of the engine or of the power cylinder-piston assembly means the sum of the total volumes displaced by the power pistons while they move from their bottom dead center (BDC) to their top dead center (TDC). Therefore, the effective stroke volume, which is the sum of the volumes displaced by the power pistons when they move from the point where the exhaust ports are just closed by the power piston to their TDC, is smaller than the total stroke volume. When two or more power cylinder-piston assemblies are included in the engine, the total stroke volume of the engine is the value which is obtained by multiplying the number of the power cylinder-piston assemblies by the above-defined total stroke volume of each power cylinder-piston assembly. The total stroke volume of the pump means the sum of the volumes displaced by the pump pistons while it moves from BDC to TDC during its compression stroke.